This invention relates to processes for producing propylene//propylene/olefin block copolymers and propylene/propylene/olefin block copolymers produced thereby. More particularly, the invention relates to processes for producing propylene//propylene/olefin block copolymers which comprise a segment A comprising a polymer based on propylene and a segment B comprising a propylene/olefin random copolymer and have improved bulk density, particle morphology, impact resistance, stiffness and transparency, using a catalyst system for an olefin polymerization comprising as a main component a metallocene catalyst having an enhanced polymerization activity, and propylene//propylene/olefin block copolymers produced using the catalyst system.
Propylene//propylene/olefin block copolymers comprising a polymer segment based on propylene and a propylene/olefin random copolymer segment are excellent in stiffness and impact resistance, and have been used as propylene type copolymers for various moldings in the fields of automobiles, electric home appliances, large size sundry goods or the like.
Such propylene//propylene/olefin block copolymers have been produced by polymerizing propylene with propylene and other olefin(s) than propylene in the presence of a catalyst system which comprises an inorganic transition metal compound catalyst having titanium trichloride, titanium tetrachloride or the mixture thereof supported on a support such as magnesium chloride or the like, in combination with an organoaluminum compound, what is called a Ziegler-Natta catalyst system.
In recent years, many processes of producing propylene//propylene/olefin block copolymers have been proposed which comprise using metallocene catalyst systems comprising an organic transition metal compound having at least one xcfx80-electron conjugated ligand, i.e., a metallocene compound in combination with aluminoxane, in place of prior Ziegler-Natta catalyst system. Propylene//propylene/olefin block copolymers produced using these metallocene catalyst systems have various expected uses, because of the balance of stiffness and impact resistance being more improved than block copolymers produced using prior Ziegler-Natta catalyst system.
JPA-4-337308 discloses a process for the production of a propylene//propylene/olefin block copolymer which comprises polymerizing propylene alone or a propylene/ethylene mixture containing up to 6% by weight of ethylene, followed by copolymerizing so as to provide a weight ratio of ethylene units/propylene units in the polymer chain being from 10/90 to 95/5, in the presence of a homogeneous catalyst system comprising a cyclopentadienyl group-containing transition metal compound represented by a specific formula and an organoaluminum compound. JPA-5-202152 and JPA-6-206921 disclose processes for the production of polypropylene molding materials wherein a liquid propylene is polymerized in the first stage in the presence of a homogeneous catalyst system comprising an indenyl group-containing transition metal compound represented by a specific formula and an organoaluminum compound, to produce a crystalline polymer containing at least 95% by weight of polypropylene, and in the second stage propylene and ethylene are copolymerized in solution or in suspension (JPA-5-202152) and in gas phase (JPA-6-206921) in the presence of ethylene, to produce a non-crystalline ethylene/propylene copolymer containing 20-90% by weight of ethylene.
JPA-8-92337 discloses a block copolymer comprising a propylene polymer block with a propylene content of 100-80% by weight and an ethylene content of 0-20% by weight, and a copolymer block with a propylene content of 0-99.9% by weight, an ethylene content of 99.99-0.09% by weight and a polymerizable polyene compound of 0.01-60% by weight. The block copolymer is produced by carrying out the sequential steps of forming the propylene polymer block and forming the copolymer block, in the presence of a homogeneous catalyst system which comprises a transition metal compound catalyst including an inorganic titanium compound and various metallocene compounds and also a compound which reacts with the transition metal compound to form an ion complex, or a homogeneous catalyst system wherein an organometallic compound including an organoaluminum compound is further incorporated in said catalyst system.
JPA-9-316147 discloses a propylene//ethylene/xcex1-olefin block copolymer wherein a polypropylene component and a copolymer component of ethylene/xcex1-olefin of C4 or more are block-copolymerized. The block copolymer is produced by polymerizing the polypropylene component and the ethylene/xcex1-olefin copolymer component in the presence of a homogeneous catalyst system which comprises a transition metal compound having a cyclopentadienyl ring, in combination with at least one compound selected from aluminoxane, a compound which reacts with the transition metal compound to form a stable anion and an organoaluminum compound.
JPA-6-287257 discloses a propylene//propylene/ethylene block copolymer produced by a process wherein propylene alone or propylene and ethylene are polymerized in the first stage under the condition of a propylene concentration in gas phase being 90 mol % or more, and in the second stage propylene and ethylene are copolymerized under the condition of a propylene concentration being less than 90 mol %, in the presence of a catalyst system which comprises a metallocene type transition metal compound, a product prepared by contacting clay, a clay mineral or an ion-exchange lamellar compound and an organoaluminum compound, and if necessary an organoaluminum compound.
JPA-6-172414 discloses a process for the production of a propylene//propylene/ethylene block copolymer which comprises carrying out a first polymerization wherein propylene alone or propylene and ethylene are polymerized in substantially gas phase in the presence of a catalyst system having a cyclopentadienyl ring-containing transition metal compound of a specific formula and aluminoxane supported on an organic porous polymer support, to produce a crystalline homopolymer of propylene or a copolymer of propylene and ethylene with an ethylene content of 5% by weight or less, and a second polymerization wherein propylene, ethylene and at least one comonomer selected from xcex1-olefins of C4-C20 are copolymerized so as to provide a polymerization molar ratio of propylene/comonomer being from 0/100 to 80/20.
JPA-8-27237 discloses a process of producing an ethylene/propylene//propylene copolymer which comprises carrying out the polymerization step (1) wherein propylene and at least one comonomer selected from ethylene and xcex1-olefins of C4-C20 are copolymerized so that a polymerization ratio in a molar ratio of propylene/comonomer will be in the range from 0/100 to 80/20, in the presence of at least one compound selected from a transition metal compound having a xcfx80-conjugated 5-membered ring ligand, aluminoxane, a reaction product of a boron compound and an organoaluminum compound, a Lewis acid and an ionic compound, more specifically a catalyst system having an indenyl series zirconocene and aluminoxane supported on a porous polypropylene support, and then carrying out the polymerization step (2) in the presence of the above catalyst and the polymer prepared in (1) to produce a crystalline propylene homopolymer or a propylene copolymer of propylene with at least one comonomer selected from ethylene and xcex1-olefins of C4-C20, the copolymer having a comonomer content of 10% by weight or less.
The above-described references state that the propylene//propylene/olefin block copolymers produced therein have well-balanced stiffness and impact resistance at ordinary temperature and low temperature, the stiffness being evaluated by flexural modulus, etc., and the impact resistance being evaluated by Izod impact strength, etc.
JPA-6-172414 and JPA-8-27237 state that the block copolymers produced therein are improved in particle morphology and bulk density.
Of the above-described techniques, the technique using the homogeneous catalyst is applicable to a solution polymerization process, when industrial production of a propylene//propylene/olefin block copolymer is taken into consideration. However, the application of this technique to a gas phase polymerization process and a slurry polymerization process produces finely divided polymers having an extremely low bulk density. Accordingly, industrial production of a propylene//propylene/olefin block copolymer by both processes is difficult practically. In the process using a supported metallocene catalyst, a polymerization activity of the catalyst improves to some extent, the amount of aluminoxane used as a cocatalyst is reduced, and an improvement in bulk density and particle morphology of the resultant block copolymer is secured. However, further improvement in every aspect has been desired. When an application to a gas phase polymerization is especially taken into consideration, further improvements in polymerization activity of the catalyst, bulk density and particle morphology of the resultant copolymer are demanded.
An object of the present invention is to provide a process for producing a propylene//propylene/olefin block copolymer, which improves a polymerization activity of the catalyst, can reduce the amount of aluminoxane used and produces the block copolymer having improved bulk density and particle morphology, and further, excellent impact strength, stiffness and transparency, and also to provide the propylene//propylene/olefin block copolymer produced by said process.
The present inventors have investigated zealously in an effort to achieve the above object, and found that the use of a catalyst system comprising a specific supported metallocene catalyst and an organoaluminum compound in the two-stage polymerization process of a propylene//propylene/olefin as disclosed in the above references can improve the polymerization activity of the catalyst, and can produce the propylene//propylene/olefin block copolymer having improved bulk density and particle morphology, and further, excellent impact strength, stiffness and transparency, thus leading to completion of the present invention.
The present invention provides a process for producing a propylene//propylene/olefin block copolymer which comprises 20-95% by weight of segment A comprising a polymer based on propylene and 80-5% by weight of segment B comprising a propylene/olefin random copolymer, characterized by carrying out in sequence the following steps (A) and (B):
(A) a first polymerization step wherein propylene or a mixture of propylene and other olefin(s) than propylene is polymerized in the presence of a catalyst system for olefin polymerization comprising a supported metallocene catalyst and an organoaluminum compound, to produce the segment A comprising a polymer based on propylene wherein a weight ratio of units of other olefin(s) than propylene/units of propylene in the polymer chain is from 0/100 to 30/70, and
(B) a second polymerization step wherein a mixture of propylene,and other olefin(s) than propylene is copolymerized in the presence of the polymer based on propylene containing the catalyst system for olefin polymerization from the first polymerization step, to produce the segment B comprising the propylene/olefin random copolymer wherein a weight ratio of units of propylene/units of other olefin(s) than propylene in the polymer chain is from 5/95 to 95/5,
the supported metallocene catalyst comprising a solid product which is prepared by carrying out in sequence the following steps of:
(a) reacting an aluminoxane with an organic transition metal compound having two xcfx80-electron conjugated ligands crosslinked each other in an inert solvent,
(b) contacting a reaction product formed in step (a) with an inorganic finely particulate support in an inert solvent at a temperature of 85-150xc2x0 C., and
(c) washing at least two times a slurry containing the solid product formed in step (b) with an aliphatic hydrocarbon at a temperature of xe2x88x9250 to 50xc2x0 C. The invention also provides the propylene//propylene/olefin block copolymer produced by said process.
Another aspect of the present invention is a process for producing the propylene//propylene/olefin block copolymer in the presence of a preactivated catalyst, instead of the supported metallocene catalyst, and also the propylene//propylene/olefin block copolymer produced by said process, the preactivated catalyst being characterized by comprising a granular product which is prepared by carrying out, subsequent to the above step (c), further step (d) wherein the supported metallocene catalyst prepared in step (c) is contacted with olefin(s) to prepolymerize the olefin(s) and 0.01-100 kg of the olefin prepolymer per kg of the supported metallocene catalyst is further supported.